This invention relates to communication systems using multicarrier modulation. More particularly, the invention relates to multicarrier communication systems that prioritize communication data streams.
The public switched telephone network (PSTN) provides a widely available form of electronic communication. In part because of its ready availability and low facilities cost, the PSTN is carrying increasing amounts of high rate data transmissions. Data here is understood to represent all forms of digitally encoded communication information including internet, Asynchronous Transfer Mode, voice, and video data and the like. Structured originally to provide voice communication with its narrow bandwidth requirements, the PSTN increasingly relies on digital systems to meet the demand for high transmission rates.
A major limiting factor in the ability to implement high-rate digital transmission has been the subscriber loop between the telephone central office (CO) and the customer premises equipment (CPE). This loop commonly comprises a single pair of twisted copper wires, which are well suited for carrying low-frequency voice signals for which a bandwidth of 0-4 kHz is adequate. However, this loop does not readily accommodate broadband communication requirements (i.e., bandwidths on the order of hundreds of kilohertz or more) without adopting new techniques for communication.
One approach to this problem has been the development of discrete multitone digital subscriber line (DMT DSL) technology. This and other forms of DMT-based DSL technology (such as ADSL, HDSL, etc.) are referred to as xe2x80x9cDSL technologyxe2x80x9d or simply xe2x80x9cDSLxe2x80x9d. The operation of discrete multitone systems and their application to DSL technology is discussed more fully in xe2x80x9cMulticarrier Modulation For Data Transmission: An idea whose Time Has Come.xe2x80x9d IEEE Communications Magazine, May 1990, pp. 5-14.
The device that transmits and receives data, built according to DSL technology, is often referred to as a DSL transceiver. The DSL transceivers used at the central office and at the customer premises site are referred to as the CO DSL transceiver and the CPE DSL transceiver, respectively.
In DSL technology, communication over the local subscriber loop between the CO transceiver and the CPE transceiver, is accomplished by modulating the data to be transmitted onto a multiplicity of discrete frequency carriers which are summed together and then transmitted over the subscriber loop. Individually, the carriers form discrete, non-overlapping communication subchannels of limited bandwidth. Collectively, the carriers form a broadband communication channel. At the receiver end, the carriers are demodulated and the data is recovered.
Each subchannel carries a number of bits which may vary from subchannel to subchannel, depending on, for example, the signal-to-noise ratio (SNR) of the individual subchannel. The aggregate communication rate may also vary for different subscriber loops and different communication conditions. The number of bits that can be accommodated under a specified set of communication conditions is known as the xe2x80x9cbit allocationxe2x80x9d of the subchannel.
The SNR of the respective subchannels is determined by transmitting a reference signal over the various subchannels and measuring the SNR of the received signal. The loading parameters (including the number of bits allocated to each subchannel and the subchannel gains) are typically calculated at the receiving (xe2x80x9clocalxe2x80x9d) end of the subscriber line (e.g., at the CPE transceiver in the case of transmissions from the telephone central office to the subscriber, and at the CO transceiver in the case of transmissions from the subscriber premises to the central office). These parameters are communicated to the transmitting (xe2x80x9cremotexe2x80x9d) end so that each transmitter-receiver pair uses the same communication parameters. The bit allocation and subchannel gain parameters are stored at both ends of the communication pair link for use in defining the number of bits to be used on the respective subchannels in transmitting data to a particular receiver. Other subchannel parameters such as time and frequency domain equalizer coefficients may also be stored to aid in defining the subchannel communication.
The Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T) approved two DSL communication system standards, xe2x80x9cAsymmetrical Digital Subscriber Line (ADSL) Transceiverxe2x80x9d, documented in Draft Recommendation G.992.1, Melbourne, Australia, Mar. 29, 1999, and xe2x80x9cSplitterless Asymmetrical Digital Subscriber Line (ADSL) Transceiverxe2x80x9d, documented in Draft Recommendation G.992.2, Feb. 17, 1999. Both of these standards specify that the transceiver utilizes DMT-based modulation to transmit data over traditional telephone lines, referred to as xe2x80x9cplain old telephone servicexe2x80x9d or POTS. In these Recommendations, the data rate of communication from the CO transceiver to CPE transceiver, called downstream communication, is different from the data rate from the CPE transceiver to the CO transceiver, called upstream communication. The transceiver built according to the G.992.1 Recommendation is traditionally referred to as the full-rate ADSL transceiver, or G.dmt transceiver; the transceiver built according to the G.992.2 recommendation is traditionally referred to as the G.lite transceiver. A significant portion of the G.lite transceiver is a subset of the G.dmt transceiver.
Both G.dmt and G.lite transceivers use communication principles common to other DMT-based DSL transceivers, namely data is communicated using non-overlapping subchannels, each of which can carry a different number of bits based on its SNR measurement.
During the transceiver initialization phase, a channel analysis is performed to determine the bit allocation of each subchannel. The channel analysis determines the maximum number of bits that each subchannel can carry based on the channel SNR measurement, the required minimum SNR margin (which is determined by the maximum tolerated bit error rate), and the coding gain. The coding gain, obtained by using Reed-Solomon Coding or Trellis Coding methods and measured in decibels, can increase the total number of bits available per unit of modulated information given the maximum tolerated bit error rate. After channel analysis, the maximum aggregate communication rates for the specified communication conditions are obtained for both downstream and upstream communications. Gathered by the CO transceiver and communicated to the CPE transceiver in a specified message, this data is used to define the rate options available to both the CO transceiver and CPE transceiver. As specified in the G.992.1 and G.992.2 Recommendations, the CO transceiver provides four downstream and four upstream rate options which can be employed by the CPE and CO transceivers respectively.
To support various applications, as specified in the G.992.1 Recommendation, the G.dmt transceiver supports up to seven downstream bearer channels, and up to three upstream bearer channels, where a bearer channel is defined as a user communication stream of data communicated at a specified communication rate that is communicated without modification by an ADSL transceiver.
A G.dmt transceiver can also transmit data in two different latency paths. The first path is called the fast data path which is used for delay sensitive applications, such as voice telephony. The second path is called the interleaved data path which is used for error sensitive applications, such as file transfer applications and video applications which are more affected by transmission errors than by transmission delays. To accommodate the framing requirements of different applications, the G.992.1 Recommendation also specifies four framing modes each of which has a different amount of overhead framing bits.
Given the complexity of multiple framing modes, multiple bearer channels, and dual latency paths, there exists a need for a process to generate a set of rate options for either downstream or upstream communication. There is a further need that this process effectively allocate the total available bits carried by a DMT symbol to different bearer channels according to the given channel conditions and supported operation modes.
The present invention features a system and a method that dedicates communication bandwidth to at least one prioritized bearer channel. In one aspect the invention features a method used in multicarrier communications between transceivers. According to the invention, the number of bits contained in a discrete multitone symbol is determined, prioritization information regarding the at least one bearer channel is received, and portions of the discrete multitone symbol are allocated to the at least one bearer channel based on the prioritization.
In one embodiment, the at least one bearer channel has an associated maximum number of bits. When portions of the discrete multitone symbol are allocated to the at least one bearer channel, that bearer channel is allocated its maximum number of bits. The allocation procedure is done in the order of the priority of the at least one bearer channel and continues as long as bandwidth is available in the discrete multitone symbol.
In another embodiment, the at least one bearer channel has an associated minimum number of bits. When portions of the discrete multitone symbol are allocated to the at least one bearer channel, that bearer channel is allocated at least its minimum number of bits.
In yet another embodiment, the number of overhead error correction check bytes is determined.
In still another embodiment, an input parameter designating a maximum number of bits supported given a specified coding gain is received. In other embodiments, input parameters designating a framing mode, a maximum interleave depth, a maximum number of error correction check bytes, a latency path, a maximum allowed delay for communications between transceivers in the multicarrier communication system, and a number of subchannel carriers in the discrete multitone symbol carrying bits are received.
In another aspect, the invention features a method for dedicating bandwidth to a plurality of prioritized bearer channels. The number of bits contained in a discrete multitone symbol modulated by a modulator are determined. Prioritization information regarding the plurality of bearer channels is received. In addition the number of bytes allocated to each of the plurality of bearer channels using a fast data path and an interleaved data path is determined based on the prioritization. Further, the number of check bytes per discrete multitone symbol for the fast and the interleaved data paths is determined. Also determined are the number of frames per interleaved codeword and an interleave depth for the interleaved data path.
In another aspect, the invention features a multicarrier communication system having at least one bearer channel and a transceiver dedicating bandwidth to the at least one bearer channel. The transceiver includes a modulator that modulates discrete multitone symbols and a rate option generator. The rate option generator determines the number of bits contained in the discrete multitone symbol and receives prioritization information regarding the at least one bearer channel. In addition the rate option generator allocates a portion of the number of bits contained in the discrete multitone symbol to the at least one bearer channel based on the prioritization.
In one embodiment the at least one bearer channel has an associated maximum number of bits. When portions of the discrete multitone symbol are allocated to the at least one bearer channel, the rate option generator allocates to each bearer channel its maximum number of bits. The allocation procedure is done in the order of the priority of the at least one bearer channel and continues as long as bandwidth is available in the discrete multitone symbol.
In another embodiment, the at least one bearer channel has an associated minimum number of bits. When portions of the discrete multitone symbol are allocated to the at least one bearer channel, the rate option generator allocates to each bearer channel at least its minimum number of bits.
In yet another embodiment the rate option generator determines a number of overhead error correction check bytes.
In still another embodiment the rate option generator receives an input parameter designating a maximum number of bits supported given a specified coding gain. In other embodiments the rate option generator receives input parameters designating a framing mode, a maximum interleave depth, a maximum number of error correction check bytes, a latency path, a maximum allowed delay for communications between transceivers in the multicarrier communication system, and a number of subchannel carriers carrying bits in the discrete multitone symbol.
In another aspect the multicarrier communication system dedicates communication bandwidth to a plurality of prioritized bearer channels. In one aspect, a rate option generator determines the number of bits contained in a discrete multitone symbol. The discrete multitone symbol is modulated by a modulator. In addition the rate option generator receives prioritization information regarding the plurality of bearer channels. This prioritization information is used by the rate option generator to determine the number of bytes to allocate to each of the plurality of bearer channels using a fast data bath. The prioritization information is also used by the rate option generator to determine the number of bytes to allocate to each of the plurality of bearer channels using an interleaved data path. Further, the rate option generator determines the number of check bytes per discrete multitone symbol for the fast data path and the number of check bytes per discrete multitone symbol for the interleaved data path. Also, the rate option generator determines the number of frames per interleaved codeword and the interleave depth for the interleaved data path.